The action movie will star Stallone, Jet Li, Randy Couture, Dolph Lundgren and Jason Statham as a band of mercenaries dispatched to overthrow a South American dictator. Whitaker is expected to take on the role of a CIA operative who liaises with the group.
According to production company Nu Image/Millennium Films, rumours that Stallone is courting his Demolition Man co-star Sandra Bullock for a part in the film are untrue.
The Expendables marks Stallone's first directorial effort since last year's Rambo.
Whitaker recently appeared in Street Kings, Vantage Point and The Last King Of Scotland, for which he won an Oscar.